NetNews Usenet Archive 1992 #18/NN_1992_18.iso/spool/comp/unix/shell/3556

Text File | 1992-08-21 | 2.5 KB | 69 lines

Newsgroups: comp.unix.shell
Path: sparky!uunet!cis.ohio-state.edu!zaphod.mps.ohio-state.edu!sdd.hp.com!mips!mips!munnari.oz.au!metro!physiol.su.OZ.AU!john
From: [e-mail] (John Mackin)
Subject: Re: Sed with shell variables
Message-ID: <[e-mail]>
Summary: Incorrect solution corrected
Reply-To: John Mackin <[e-mail]>
Organization: Department of Physiology, University of Sydney, Australia
References: <[e-mail]> <[e-mail]>
Date: Fri, 21 Aug 1992 01:36:04 GMT
Lines: 56

In article <[e-mail]>,
    [e-mail] (Thomas Koenig) writes:

> [e-mail] (Robert Gasch) writes:
>
>>The problem I ran into is that I can't get shell variables to
>>work as arguments. When I type
>>    echo $something|sed 's/$this_pattern_var/$that_pattern_var/'
>
>>sed throws up on me. I've tried various combinations of quotation
>>marks without success. Is there an easy way of doing this??
>
> echo $something | sed -e s/$this_pattern/$that_pattern/

Unfortunately, Thomas's solution is wrong.  Since Robert doesn't
say what shell he is concerned about, we can only suppose that he
wants a solution for a `conventional' shell, such as sh or csh
(and not, for example, rc, where Thomas's solution would be right,
but for the wrong reasons).  These shells are macro languages that
re-scan their arguments for delimiters (usually, white space)
after variable substitution.  So the solution presented above
will only work as long as the values of the two pattern variables
don't contain any whitespace (space or tab characters).

Next time, Robert, please do say what shell you are asking about.
I hope, though, that the same solution is correct in this instance
for both sh and csh.  I know it is correct in sh.

Worked example (": mod; " is the prompt):

#
# Set variables
#
: mod; from='from pat'
: mod; to='to pat'

#
# Demonstrate that Thomas's idea does not suffice
#
: mod; echo "from pat to bill it's all a thrill" | sed s/$from/$to/
command garbled: s/from

#
# Give the correct solution
#
: mod; echo "from pat to bill it's all a thrill" | sed "s/$from/$to/"
to pat to bill it's all a thrill

That's the way double-quotes work in sh (and, I think, in csh).
Great, isn't it?  Change to rc now, while you're still impressionable
(you _must_ be impressionable, I hope, if you didn't know how to
do this), and reap the benefit of _only one kind of quoting_!!!

--
John Mackin <[e-mail]>
        case '\7': *((char *)(384*1024L+062)) = 2;      /* beep */